Protective coating for molybdenum



July 25, 1961 H. E. GRENOBLE 2,993,264

PROTECTIVE COATING FOR MOLYBDENUM Filed Dec. 23, 1955 F .2. 35000 lgInventor.- Henbert E. Grenob/e,

///'s Attorney.

Unite York Filed Dec. 23, 1955, Ser. No. 554,999 4 Claims. (Cl. 29-1835)This invention relates to the provision of protective coatings forstructural components composed of molybdenum and molybdenum base alloysand more particularly to coatings for articles of molybdenum andmolybdenum-rich alloys which function to resist oxidation, to preventoxidation of the molybdenum article, and which are resistant toattrition by abrasion and erosion.

Molybdenum and alloys of which molybdenum is the principal constituentare known to have high strength at elevate-d temperatures, and exceptfor the fact that these materials are subject to drastic corrosion oroxidation when exposed to oxidizing atmospheres at temperatures over1400 F., would be excellent as a structural material for the fabricationof components subjected to high temperature and high stress in gasturbine applications. It has been proposed that such components as gasturbine buckets, blades and the like might be successfully fabricatedfrom molybdenum and molybdenumrich alloys if the external surfacesexposed to oxidation could be provided with a protective coating toexclude the oxidizing atmosphere.

Many different coating materials have been applied to molybdenumarticles in an attempt to prevent this high temperature oxidation, butwhile it has been found a relatively easy matter to provide a stable,continuous coating which will exclude oxygen for long periods of timeunder static conditions, when the molybdenum article is flexed orotherwise subjected to oscillatory loads to produce reverse bending,these previously known coatings have been effective for only briefperiods of time.

A principal object of my invention is the provision of stable, adherentcoatings for articles made from molybdenum and molybdenum-rich alloyswhich resist oxidation and penetration of oxidizing atmospheres for longperiods of time at elevated temperatures. An additional object of myinvention is the provision of composite articles of molybdenum and ofmolybdenum-rich alloys which are capable of withstanding oscillatoryloading in oxidiz' ing atmospheres at temperatures in excess of 1500 F.for extended periods of time. A still further object of my invention isto provide a method and apparatus for cladding molybdenum andmolybdenum-rich articles in order to increase their resistance tooxidation, abrasion and erosion.

Briefly stated, in accordance with one aspect of my invention I providea molybdenum or molybdenum-rich alloy article having a plurality ofelectrodeposited, alternately disposed layers of chromium and nickelwhich may advantageously be enclosed in a preformed sheet metal jacketand a method and apparatus for preparing such an article.

My invention will be better understood from the following descriptiont-aken in conjunction with the accompanying drawing, and its scope willbe pointed out in the appended claims.

In the drawing,

FIG. 1 is a perspective view of a typical gas turbine bucket;

FIG. 2 is a graphical illustration of fatigue properties ofelectroplated articles;

FIG. 3 is a transverse, cross-sectional view of drawing apparatus bywhich sheet metal may be preformed for turbine bucket jackets;

Patent 6 ice FIG. 4 is a perspective of a preformed sheet metal jacketelement;

FIG. 5 is a longitudinal cross-section of apparatus for applyingpreformed sheet metal jacket elements to a turbine bucket;

FIG. 6 is a cross-sectional view of a jacketed bucket prior to trimmingtaken along line 6-6 of FIG. 5; and

FIG. 7 is a fragmentary sectional view of an embodiment of my invention.

As shown in FIG. 1, the typical gas turbine bucket 10 comprises a bladeor air foil portion 11, a shank 12 and dovetail portion 13. In operationin a gas turbine, the bucket 10 is usually afiixed and retained in theperiphery of a rotatable disk-like element by the dovetail and and shankportions which engage a mating slot in the disk. The disk and bucket arerotated at high rates of speed at high temperatures, of the order of1400 F.

and above, while surrounded by an oxidizing atmosphere. Extremely highstresses are developed at the dovetail and shank portions and higherstresses in the air foil section 11. Further, these stresses arecomplicated by vibrational loads imposed during rotation which tend tosubject the air foil and associated structure to bending first in onedirection and then in a reverse direction.

When molybdenum buckets were fabricated, attempts were made to protectthem from oxidation by the elec trodeposition of nickel thereon. First,great difficulty was experienced in providing an electrodepositedcoating of nickel upon molybdenum or molybdenum-base alloys which wouldadhere thereto when heated. After means had been found to provideadherent electrodeposited nickel films upon molybdenum andmolybdenum-rich alloys, I then found that these coatings were not stableat elevated temperatures when subjected to reverse bending loads, or, ascommonly referred to, fatigue loading conditions, I discovered thatunder this type of loading rapid intergranular failure occurred at thegrain boundaries of the nickel coating, resulting in cracks forming atthe nickel grain boundaries through which oxygen was able to pass andattack the molybdenum substrate. I further discovered that merelyincreasing the thickness of the nickel coating did not produce anincrease in time of oxidation resistance commensurate with the increasein coating thickness. It has been proposed that a protective coatinghaving greater resistance to penetration be formed by depositing achromium film upon the molybdenum surface, followed by anelectrodeposited nickel film thereon and followed by a finalelectrodeposited chromium film thereover, a slow heat treatment cyclefollowing the nickel deposition and a second slow heat treatment cyclefollowing the final chromium deposition. This type of coating has beenshown to have greater resistance to penetration than a coatingconsisting essentially of nickel. I have discovered, however, that acoating consisting of a plurality or multiplicity of layers of nickelinterspersed by chromium layers produces a resistance to penetration fargreater than that which would be expected.

In particular, I have discovered that a molybdenum or molybdenum-richalloy article provided with a coating having an aggregate thickness offrom about 0.005 to 0.010 inch, consisting of a film of chromium lessthan 0.001 inch thick, preferably about 0.0005 inch thick,electrodeposited upon the molybdenum alloy, followed by a plurality oflayers of electrodeposited nickel alternated with layers ofelectrodeposited chromium has outstanding resistance to fatigue failure.The layers of nickel are each preferably about 0.0006 to 0.0008 inchthick, while the intervening layers of chromium, including a final layerof chromium covering the last nickel layer, are each about 0.0002 to0.0003 inch thick. The thickness of the several coatings should beregulated so that the chemical composition of the electrodepositedcoating ranges: between about 65 to 80 percent by weight nickel, thebalance being substantially chromium. In practice, it is preferred thatthe article be subjected to a short time duration, A to /2 hour, annealat about 1800 F. in a hydrogen atmosphere after each chromium layer hasbeen applied, and to a 4- to 6 hour anneal at about 1800 F. in ahydrogen atmosphere after each nickel layer has been applied. Uponmicroscopic examination, the coating layers are found to have beenmodified by diffusion during heat treatment to form a plurality ofalternating layers of nickel-rich nickel-chromium alloy and nickel.

In order to compare the effectiveness of the multiple nickel layercoating of my invention with a coating con sisting of a nickel layerinterposed between a pair of chromium layers, a plurality of fatiguetest specimens were prepared from forged bars of a commerical alloycontaining 0.3 percent columbium, balance substantially all molybdenum.Each fatigue test specimen was prepared from a round bar of thismaterial 8% inches long having a diameter of 0.625 inch. A pair ofopposed cylindrical surface each having a inch radius was machined inthe central portion of each of the bars, the minimum distance betweenthe opposed surfaces being about 0.343 inch.

One group of the test bars was then plated in the following manner. Thesurfaces of the molybdenum alloy bars were cleaned anodically at 12volts for 1 minute in an aqueous solution of sulfuric acid at roomtemperature followed by rinsing in water, ammonium hydroxide, andfinally distilled water. The bars were then placed while still wet in aconventional chromium plating solution in which the concentration of CrOwas maintained between 275 and 325 grams per liter. The bath temperaturewas held between 65 and 70 C. and the cathode current density adjustedto 1.5 amperes p.s.i. After a layer of chromium at least 0.0003 inchthick had been applied in this manner, the bars were removed from theplating bath. The chromium plated specimens were then thoroughly washedand placed directly in a conventional acid-nickel strike bath for 5minutes at a cathode current density of 100 milliamperes per squarecentimeter. The specimens where then transferred directly withoutrinsing to a conventional Watts-type nickel plating solution whereplating was continued for 5 minutes. At the end of this time thespecimens were rinsed and dried and heat treated for 15 minutes in ahydrogen atmosphere from about 1700 to 1850 F., preferably 1800 F.

Following heat treatment, the specimens were electropolished in aphosphoric acid-sulfuric acid solution, rinsed, returned to the nickelstrike bath for 5 minutes and again transferred directly to the Wattssolution, where electroplating of nickel was continued until about 0.003inch total thickness of nickel was attained. Following the nickelplating step, the specimens were heat treated for at least 15 minutes atabout 1800" F., in a hydrogen atmosphere. This heat treatment wasfollowed by cleaning the specimens by electropolishing in the phosphoricacid-sulfuric acid solution for a few seconds, followed by rinsing indistilled water. A final chromium plated film about 0.0003 inch thickwas applied over the nickel layer by utilizing the same bath andprocedure recited for the first chromium coating and the resultingspecimen heat treated for at least 15 minutes at 1800 F. in a hydrogenatmosphere.

A second group of test specimens were electroplated and heat treatedaccording to my invention, namely, by first applying a chromium layer ofabout 0.0005 inch thickness followed by a preliminary nickel film, heattreated, electroploished, plated with nickel to about 0.0007 inchthickness, heat treated, electropolished, plated with a second film ofchromium about 0.0003 inch'thick,

given a preliminary nickel plate, heat-treated,- electro-- polished,plated to form a second nickel film of about 0.0007 inch thickness, heattreated, electropolished, plated with a third film of chromium about0.0003 inch thick, given a preliminary nickel plate, heat treated,electropolished, plated to form a third nickel film about 0.0007 inchthick, heat treated, electropolished, plated with a fourth film ofchromium about 0.0003 inch thick and heat treated. In preparing thesurfaces of these specimens for plating, heat treatment, cleaning ofplated surfaces, and plating, the same procedurm and solutions used inthe preparation of the first group of specimens were employed.

The several solutions used are quite conventional and the compositionsthereof may be varied considerably within the scale of the plating art.However, the chromium plating bath actually used consisted of an aqueoussolution containing from about 275 to 325 grams per liter C10 andsufficient sulfuric acid so that the ratio of CrO to sulfate ions wasmaintained at about 100 to l. The acid nickel sulfate strike bathcontained about 450 grams per liter of nickel sulfate, about 50 gramsper liter concentrated sulfuric acid, and the balance water. The Wattssolution contained about 280 to 300 grams per liter nickel sulfate, 30grams per liter nickel chloride, 40 grams per liter boric acid, and thebalance water. The electropolishing solution containing about 20 percentby volume concentrated sulfuric acid, balance concentrated phosphoricacid.

It may be seen from the foregoing that the first group of specimens wereprovided with a plated coating consisting of a 0.0003 inch thicknesslayer of chromium, a 0.003 inch thick layer of nickel thereon, and afinal 0.0003 inch thick layer of chromium. The second group of specimenswere provided with a multiple layer coating consisting of a 0.0005 inchthick layer of chromium followed by a 0.0007 inch thick layer of nickel,a 0.0003 inch thick layer of chromium, a 0.0007 inch thick layer ofnickel, a 0.0003 inch thick layer of chromium, a 0.0007 inch thick layerof nickel, and a 0.0003 inch thick layer of chromium.

These specimens were then tested for resistance to fatigue loading atelevated temperatures in an air atmosphere in the conventional manner,and representative results of these tests are reproduced in thefollowing table:

Gr-Ni- Cr PLATED SPEOIMENS 1 Test terminated, specimen unbroken.

In the foregoing table, specimens from the first group are identified asCrNiCr Plated Specimens, while the specimens from the second group areidentified as Multiple Layer Plated Specimens. It will be immediatelyapparent that the test bars coated according to my invention were ableto withstand high temperature fatigue loads for much longer times thanthe chromium-nickelchromium plated comparison specimens. For example, at1650 F. the limiting stress for over 1 million cycles is about 25,000p.s.i. for the chromium-nickel-chromium coated bars, while it is about30,000 p.s.i. for the mul tiple layer plated bars'of my invention.

in FIG. 2 in which curve 15 shows the fatigue strength of the test barscomprising my invention and the curve 16 shows the fatigue strength ofthe chromium-nickelchromium test bars. It should be noted that theordinate axis of the graph is a linear scale, while the abscissa axis ofthe graph is composed of recurrent cycles of logarithmic scales.

Microscopic examination of cross-sections of fatigue tested specimens ofmy invention revealed that while the nickel zones or layers of thecoating still exhibited cracks at the grain boundaries, thenickel-chromium alloy zones acted to prevent these small cracks fromgrowing longer and forming a single crack extending through the entireprotective coating.

When gas turbine buckets were plated according to my invention andinstalled in an aircraft gas turbine, it was found that the stream ofcombustion gases passing over the buckets contained abrasive particleswhich abraded or eroded the protective coating excessively. Accordingly,I have provided the following apparatus for cladding turbine buckets andthe like with sheet metal.

The apparatus shown in cross-section in FIG. 3 comprises a forming diein which a base or punch member 17 is provided with a contoured uppersurface 18 adapted to receive and conform to one side of a turbinebucket 10. Punch member 17 rests upon a supporting platen 19 of asuitable press apparatus, a resilient pad 20 surrounds punch 17 andsupports a spacer element 21 and a blank holder 22. A suitablydimensioned sheet of cladding metal 23 is supported upon blank holder 22and a resilient or deformable die 24 constructed of rubber or the likeis adapted to be pressed against the upper surfaces of bucket 10 andpunch 17 by the upper platen 25.

When the platen 25 is urged toward platen 19, the sheet metal blank 23is deformed or drawn about the upper contours of the bucket 10 and theexposed upper surfaces of punch 17 to form a sheet having substantiallythe exact configuration of one-half of bucket 10 embossed thereon, asshown in FIG. 4 at 26. Further, coaction between die 24 and blank holder22 during the forming operation produces a rimlike flat margin 27 at theedges of the formed sheet having a useful function to be describedsubsequently.

It may be seen that by removing bucket 10 from punch 17 and removingspacer element 21 that a second sheet of cladding metal 23' not shown inFIG. 3 may be supported upon blank holder 22 and deformed or drawn overthe contoured surface 18 of die 17 to produce an embossed surfacethereon contoured to substantially the exact configuration of the otherside of bucket 10.

These contoured sheets 23 and 23 may then be bonded to the surfaces ofbucket 10 by means of the following apparatus. As was shown in FIG. apressure chamber 27 is provided having recess means 28 about itsperiphery to accommodate the edge portions 27 of contoured sheets 23 and23. Sheets 23 and 23' are assembled with a bucket and edge portions 27thereof are sealed in recesses 28 by means of resilient gasket elementsor the like 29. As shown, the sheets 23 and 23' act as a dia phragm toseparate the interior of pressure chamber 27 into two substantiallyequal volume closed compartments. Means are provided for heating theinterior of the chamber and assembly 23, 23' and '10, such as, forexample, electrical heating elements 30.

Means are provided at 31 for the introduction of hydrogen between sheets23 and 23 and around bucket 10 and exiting at 32. Means 33 are alsoprovided for introducing an inert gas such as argon into the interior ofthe chamber 27 at high but equal pressures to each of the twocompartments thereof.

As a specific example of the operation of the apparatus of my invention,assume that contoured sheets 23 and 23' have been formed from 0.005 inchthick nickel sheet metal. The formed sheets 23 and 23' are assembledwith 6 bucket 10 as shown in FIG. 5 and hydrogen gas is passed throughthe interior of the assembly as it is heated to a temperature of about2000 to 2400 F., preferably about 2200 F. After the assembly has reachedthe desired temperature, an inert gas, for example argon, is admitted tothe two chambers until a pressure of about 600 p.s.i. or greater isattained therewithin. The flow of hydrogen is terminated and thetemperature and chamber pressure are maintained for a time, of the orderof 12 hour to 2 hours, sufficient to pressure weld the contoured sheets23 and 23' to the bucket 10. The relationship of sheets 23 and 23 tobucket 10 is shown in FIG. 6. After cooling, the excess sheet metal istrimmed from the periphery of the clad bucket, for example, by cutting23 and 23 at 34 and 35.

While in the foregoing example the sheets 23 and 23 have been disclosedas being formed from 0.005 inch thick nickel sheet, it is obvious thatthicker or thinner sheet may be successfully used or that sheet metalcomposed of nickel-base alloys or other deformable alloys havingsuitable corrosion and welding characteristics may be used. For example,0.0035 inch thick sheet metal composed of about 80 percent nickel, 13percent chromium, 6.5 percent iron and 0.20 percent copper has beensuccessfully deformed and pressure welded to buckets by means of theforegoing apparatus. While molybdenum articles may be directly clad withsheet metal in this manner, it is desirable and preferred thatmolybdenum turbine buckets be protected from oxidation by the platedcoating of my invention and that the plated coating be in turn protectedfrom abrasion by the provision of sheet metal cladding thereover,pressure welded to the outer layer of the plated coating. In thismanner, if there are any small openings in the sheet metal jacket,particularly where formed sheet 23 joins sheet 23, the underlying platedcoating will prevent the drastic oxidation of the molybdenum ormolybdenum-base alloy of the bucket. Such a preferred construction isillustrated in FIG. 7 in which bucket 10 is provided withelectrodeposited layers of chromium 36, 37, 38 and 39, electrodepositedlayers of nickel 40, 41 and 42 and a sheet metal jacket 23 pressurewelded to chromium layer 39. Of course, the various plating operations,heat treatment of the plated layers, and the forming and pressureWelding of the sheet metal jacket are preferably accomplished as recitedpreviously.

It should be further noted that while the specific examples of myinvention described previously have been restricted to a multiple-layercoating consisting of four layers of chromium and three layers ofnickel, a greater number of alternating layers may be deposited ifdesired. For example, I have deposited a multiple-layer consisting of asmany as seven layers of chromium separated by six 7 alternate layers ofnickel upon gas turbine buckets without adversely afiecting the qualityof the coating. Preferably at least four layers of chromium and threelayers of nickel should be employed and the aggregate composition ofthis preferred coating and coatings consisting of a greater number ofalternately disposed layers should consist of about 65 to 80 weightpercent nickel, and the balance chromium.

From the foregoing, it may be readily seen that I have provided anoxidation resistant, electroplated coating for molybdenum andmolybdenum-base alloys which is particularly resistant to failure atelevated temperatures and high fatigue loads. Further, I have provided asheet metal jacket for protecting such a coating from abrasion,particularly where the coated article is a gas turbine bucket and haveprovided apparatus particularly adapted to forming and pressure weldingthe jacket elements to the plated article. While specific examples of myinvention have been recited in the foregoing specification and shown inthe drawing, it will be obvious to those skilled in the art that variouschanges and modifications may be made Without departing from theinvention, and it is intended to cover in the appended claims all suchchanges and modifications that come within the true spirit and scope ofthe invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A composite article including a body consisting substantially ofmolybdenum and a multiple layer coating substantially covering thesurface of said body to provide improved resistance against hightemperature oxidation of said body when it is subjected to cyclicstresses, said multiple layer coating comprising a first chromium layerdeposited on said body as a continuous film covering substantially theentire surface thereof, and a plurality of nickel layers of from 0.0006to 0.0008 inch thick altermating with a plurality of chromium layers offrom 0.0002 to 0.0003 inch thick, said layers being present ascontinuous films covering said first chromium layer and ending in anoutermost layer of chromium, adjoining nickel and chromium layersforming alloys at the interfaces therebetween which improve theprotective properties of said multiple layer coating.

2. A composite article as defined in claim 1 wherein the total thicknessof said multiple layer coating is from 0.005 to 0.010 inch.

3. A composite article as defined in claim 1 wherein the thickness ofsaid first chrome layer is from 0.001 to 0.0005 inch.

4. A composite article including a body consisting substantially ofmolybdenum and a multiple layer coating substantially covering thesurface of said body to provide improved resistance against hightemperature oxidation of said body when it is subjected to cyclicstresses, said multiple layer coating comprising a first chromium layerdeposited on said body as a continuous film of from about 0.001 to0.0005 inch thick covering substantially the entire surface thereof, aplurality of nickel layers of from 0.0006 to 0.0008 inch thickalternating with a plurality of chromium layers of from 0.0002 to 0.0003inch thick, said layers being present as continuous films covering saidfirst chromium layer and ending in an outermost layer of chromium, and asheet metal jacket of a corrosion resistant material pressure welded tothe outermost layer of the multiple layer coating to protect saidarticle against abrasion.

References Cited in the file of this patent UNITED STATES PATENTS1,792,638 Harrington Feb. 17, 1931 1,793,913 Detwiler Feb. 24, 19312,375,154 Volterra May 1, 1945 2,402,834 Nachtman June 25, 19462,417,133 Schweiker Mar. 11, 1947 2,683,305 Goetzel July 13, 19542,697,130 Korbelak Dec. 14, 1954 2,763,920 Turner Sept. 25, 19562,772,227 Quaely et a1 Nov. 27, 1956 2,854,739 Bartlett et al. Oct. 7,1958

1. A COMPOSITE ARTICLE INCLUDING A BODY CONSISTING SUBSTANTIALLY OFMOLYBDENUM AND A MULTIPLE LAYER COATING SUBSTANTIALLY COVERING THESURFACE OF SAID BODY TO PROVIDE IMPROVED RESISTANCE AGAINST HIGHTEMPERATURE OXIDATION OF SAID BODY WHEN IT IS SUBJECTED TO CYCLICSTRESSES, SAID MULTIPLE LAYER COATING COMPRISING A FIRST CHROMIUM LAYERDEPOSITED ON SAID BODY AS A CONTINUOUS FILM COVERING SUBSTANTIALLY THEENTIRE SURFACE THEREOF, AND A PLURALITY OF NICKEL LAYERS OF FROM 0.0006TO 0.0008 INCH THICK ALTERNATING WITH A PLURALITY OF CHROMIUM LAYERS OFFROM 0.0002 TO 0.0003 INCH THICK, SAID LAYERS BEING PRESENT ASCONTINUOUS FILMS COVERING SAID FIRST CHROMIUM LAYER AND ENDING IN ANOUTERMOST LAYER OF CHROMIUM, ADJOINING NICKEL AND CHROMIUM LAYERSFORMING ALLOYS AT THE INTERFACES THEREBETWEEN WHICH IMPROVE THEPROTECTIVE PROPERTIES OF SAID MULTIPLE LAYER COATING.